A thin film transistor (TFT) is a very important component in the technical field of display, and plays a very important role in liquid crystal display (LCD) and organic light emitting diode (OLED) devices. Presently, there are two main types of thin film transistors: bottom-gate thin film transistors and top-gate thin film transistors. Since there is no overlapping region between the gate electrode and the source/drain electrode in the top-gate thin film transistor, the stray capacitance is small and the small stray capacitance reduces the delay of the RC circuit, so that it has a higher switching speed and it is easier to achieve higher resolution display. At the same time, the number of patterning processes required for fabricating the top-gate type thin film transistor is small, and the fabricating process is simple. Therefore, top-gate thin film transistors are widely used in high-resolution OLED display devices.
In an OLED display device using the top-gate type thin film transistor, there is a problem that characteristics of driving thin film transistor and switching thin film transistor are separated, which directly affects the display effect of the OLED display device.